Year	Years
1970	3.8
1971	4
1972	4.1
1973	4.2
1974	4.3
1975	4.4
1976	4.5
1977	4.7
1978	4.8
1979	4.9
1980	5
1981	5.2
1982	5.3
1983	5.4
1984	5.6
1985	5.7
1986	5.9
1987	6
1988	6.2
1989	6.3
1990	6.5
1991	6.6
1992	6.8
1993	6.9
1994	7.1
1995	7.2
1996	7.4
1997	7.5
1998	7.7
1999	7.8
2000	8
2001	8.1
2002	8.3
2003	8.5
2004	8.6
2005	8.8
2006	8.9
2007	9.1
2008	9.2
2009	9.4